This invention relates to a method and device for operating a combined gas and steam turbine plant, and more particularly to a method and device for reducing the amount of nitrogen oxide (NOx) discharged when the plant is being stopped and started.
In stopping the combined plant using the conventional method, the gas turbine is operated at a constant load and the damper at the entrance of the heat recovery boiler is gradually closed and at the same time the by-pass damper by-passing the boiler is gradually opened to discharge the gas turbine exhaust gas into the atmosphere. That is, the amount of steam generated by the heat recovery boiler is reduced to lower the load of the steam turbine and after the steam turbine has been stopped the load of the gas turbine is lowered. In starting the plant using the conventional method, the steam from an auxiliary boiler is used to seal the glands of the steam turbine and set up a vacuum in the condenser. At the same time the load of the gas turbine is increased to its full load with the by-pass damper fully opened, after which the by-pass damper is gradually closed while the damper at the entrance of the heat recovery boiler is gradually opened to increase the amount of steam generated by the boiler and thereby increase the load of the steam turbine.
With this conventional method, however, a large amount of NOx generated when the gas turbine is operating at full load is discharged together with the turbine exhaust gas into the atmosphere through the by-pass duct without passing through a denitrating means incorporated in the heat recovery boiler such as that described in the specification of the U.S. Pat. No. 4,106,286. The conventional method also has the disadvantage that an auxilialy boiler must be provided to generate steam for sealing the glands of the steam generator. This does not agree with the principle of the combined plant that all the necessary steam is generated by utilizing the exhaust gases from the gas turbine.